With increased governmental and consumer focus on motor vehicle fuel economy, the ability of manufacturers to develop fuel efficient engines has become of paramount importance. As part of this engineering effort is the ability to accurately measure the fuel economy of such engines as errors can be costly; on either the high or the low side of measurement.
In engine test cells, the density of fuel can change 0.1% per degree Celsius. Changes of fuel density during a test measurement introduce errors in mass flow calculations. Testing has shown a definite correlation between fuel temperature stability and fuel mass flow measurement accuracy. As a result, isothermal test cell fuel systems desirably enhance accuracy substantially. Jacketed fuel lines and large in-cell fuel reservoirs are one alternative but require significant test cell space which is generally not available. Some laboratories may attempt to attain stability through long stabilization times. This effectively compromises the ability to acquire accurate fuel flow measurements during transient conditions and adversely affects test cell productivity.